Narrow-band internet-of-things (NB-IoT) is a Low Power Wide Area connectivity standard defined initially in 3GPP Release for the IoT market. NB-IoT is intended to be used for ultra-low cost user equipment (UE) devices, which are often equipped with low-cost oscillators. Such low cost oscillators may cause an initial carrier frequency offset (CFO) due to limited stability of up to 20 parts per million (ppm). IoT UE is expected to be dormant most of the time. When powered on, the UE needs to correct for these frequency offsets. Thus, the UEs need to perform time and frequency synchronization with the base station (eNodeB) in order to transmit and receive data. Synchronization may include aligning with the frequency of the base station, e.g., tuning the frequency of the UE to the frequency of the carrier signal, and aligning with the timing of the frame structure. Cell synchronization in NB-IoT is performed by the UE based on primary and secondary synchronization signals that the base station broadcasts, e.g., narrow-band primary synchronization signal (NPSS) and narrow-band secondary synchronization signal (NSSS), respectively. NB-IoT supports enhanced coverage, with the requirement to detect radio communication cells in a very low signal to noise ratio (SNR) conditions of, e.g., −12.6 dB and lower, due to difficult outdoor conditions.
Release 13 of the LTE standard defines a new UE category, NB1, in which the legacy synchronization channels, PSS and SSS, were modified to support NB-IoT with reduced bandwidth. Transmission according to the LTE standard is divided into radio frames of 10 msec. According to the LTE standard, the eNodeB repeatedly transmits a known NPSS and NSSS in constant time intervals, e.g., every 10 msec (e.g. radio frame), which the UE synchronizes with before it can receive information from the eNodeB.
Synchronization algorithms have been greatly investigated in communication and GPS systems. Known synchronization algorithms, for example, synchronization algorithms commonly used in GPS systems, are based on cross-correlation method, referred to herein as full-length cross-correlation. Full-length cross-correlation methods relay on knowledge of the synchronization signal at the receiver side. Applicable to NB-IoT systems, full-length cross-correlation methods may be implemented by correlation of the received signal at time τ denoted by x[τ] with the NPSS, e.g.:ρ(τ)=Σn=0NNPSS−1x[n+τ]NPSS*[n]exp(j2πfhn)  (Equation 1)Where NPSS* is the conjugate of the NPSS sequence (throughout the application, an asterisk,*, may indicate a conjugate value), NNPSS is the length of the NPSS sequence, τ is a candidate synchronization point in the radio frame (e.g., a point in time or a sample number within the radio frame), and fh is a hypothesis of the of the receiver's initial CFO. It is noted that the length of the full-length cross-correlation substantially equals a length of a full NPSS sequence.
Other synchronization methods, which are mentioned as suitable for NB-IoT communication systems, are based on symbol-wise auto-correlation. The symbol-wise auto-correlation relies on the repetitive structure of the NPSS sequence. NPSS sequence includes repetitions of NPSS orthogonal frequency-division multiplexing (OFDM) symbols multiplied by a known code cover. Thus, symbol-wise auto-correlation may be performed between the NPSS symbols of the NPSS sequence. Symbol-wise auto-correlation procedure may be performed based on, for example, the following equations:Sk(τ)=Σn=0Nsymbol−1x(τ+n)x*(τ+n−kNsymbol)  (Equation 2)ρ(τ)=w1S1(τ)+w2S1*(τ)S2(τ)+w3S2*(τ)S3(τ)  (Equation 3)Where Sk(τ) is the symbol-wise auto-correlation results, ρ(τ) is a cost function and w1, w2, w3 are weights.
In some scenarios UE's battery life should last over 10 years. One major contributor to the UE power consumption is the radio frequency (RF) transceiver. In order to meet the strict power requirements UE “RF-ON” duration should be minimized as possible. Reduced synchronization time results in shorter “RF-ON” period and therefore lowers the power consumption of the UE considerably.